Several positioning systems exist today, that are used to determine a three-dimensional position of an implantable positioning device in a body. These positioning systems are particularly suited for tracking the position of a cancer area for radiation therapy (radiotherapy).
One type of system is based on radio frequency transmission from an implantable positioning device acting as a transmitter to an externally arranged receiver, as described in the international patent application PCT/SE2006/001135, assigned to Micropos Medical AB, which is hereby incorporated as reference. The transmitter transmits an electromagnetic signal at a specified frequency which is received at a plurality of receiving elements in the receiver. The receiving elements are typically arranged in a plane on a treatment table under a patient in which the positioning device is implanted. The communication uses near field transmission, and magnetic coupling is used between the transmitter and receiving elements. Parameters, such as amplitude and/or phase information, are measured from all receiving elements, preferably more than three receiving elements. These measurements are used to determine the position of the implantable positioning device.
All types of positioning systems need to be tested and verified in order to estimate the 3D position of an implantable positioning device within a body. Performance verification may be achieved by arranging the implantable positioning device at one predetermined position within a phantom that has the same electrical properties as the human body tissues at the specific frequency, and arranging the phantom above the receiver. The selected parameter(s) of the received signal at each receiving element is/are measured and compared with the expected parameter(s) for the predetermined position. If no discrepancy then performance verification is achieved, and if there is a deviation for one or more receiver elements adjustment of the receiver elements is required to eliminate the deviation.
The adjustment is rather coarse, and thus the accuracy of the positioning system is limited. Furthermore, the performance verification process is time consuming and a complex process that will occupy the radiotherapy equipment at regular intervals, which in turn will reduce the number of patients that could have been treated.